Method and apparatus for setting a guard interval in an OFDM communication

ABSTRACT

A method and apparatus for setting a guard interval in an OFDM communication. The method includes attaching a part of a first valid symbol to the first valid symbol as a guard interval and attaching a part of a second valid symbol requiring higher channel quality than the first valid symbol, to the second valid symbol as a guard interval, and providing the guard interval of the second valid symbol at a length greater than the guard interval of the first valid symbol.

This is a Divisional Application of U.S. patent application Ser. No.09/464,388 filed Dec. 16, 1999 now U.S. Pat. No. 6,714,511, the contentsof which are expressly incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transmission/reception apparatus usedfor digital mobile communications using an orthogonal frequency divisionmultiplexing (hereinafter referred to as “OFDM”) system.

2. Description of the Related Art

A Conventional OFDM transmission/reception apparatus is explained usingFIG. 1 and FIG. 2. FIG. 1 is a block diagram showing an outlinedconfiguration of a transmission system of a conventional OFDMtransmission/reception apparatus. FIG. 2 is a block diagram showing anoutlined configuration of a reception system of the conventional OFDMtransmission/reception apparatus.

In FIG. 1, serial-parallel converter (hereinafter referred to as “S/Pconverter”) 11 converts a serial input signal to a plurality of parallelsignals. IDFT circuit 12 performs inverse discrete Fourier transform(hereinafter referred to as “IDFT”) on the input signals. Guard intervalinserter 13 inserts a guard interval for every valid symbol. D/Aconverter 14 performs D/A conversion on the signal with guard intervalinserted.

In FIG. 2, A/D converter 15 performs A/D conversion on a receptionsignal. Delayer 16 delays the input signal by a valid symbol length.Correlator 17 despreads the input signal. Timing generator 18 detectsthe timing of the reception signal at which the correlation resultbecomes largest. Guard interval eliminator 19 eliminates a guardinterval inserted for every symbol. DFT circuit 20 performs discreteFourier transform (hereinafter referred to as “DFT”) on the inputsignal. Coherent detectors 21 to 24 perform coherent detection on theinput signal. Deciders 25 to 28 judge the input signal. Parallel-serialconverter (hereinafter referred to as “P/S converter”) 29 converts aplurality of parallel signals to a serial signal.

Then, the operation of the conventional OFDM transmission/receptionapparatus is explained. Here, suppose the number of carriers is 4, forexample.

First, the operation of the transmission system is explained usingFIG. 1. A modulated signal input to the transmission system isS/P-converted by S/P converter 11. This results in four modulatedsignals, which are transmitted by a first, second, third and fourthcarriers, respectively.

Then, the 4 modulated signals are IDFT-processed by IDFT circuit 12.

A general OFDM transmission/reception apparatus has a frame format asshown in the frame format schematic diagram in FIG. 3. That is, in aframe format used for a general OFDM transmission/reception apparatus, asignal with the same waveform as that of the last part of a valid symbolis added at the start of the valid symbol as a guard interval. The OFDMtransmission/reception apparatus can eliminate a delayed signal with ashorter delay time than this guard interval through DFT processing ofthe reception system.

Guard interval inserter 13 inserts a guard interval into theIDFT-processed signal. The signal with the guard interval inserted isconverted to an analog signal by D/A converter 14. In this way, atransmission signal is obtained.

Then, the operation of the reception system is explained using FIG. 2.The reception signal input to the reception system is converted to adigital signal by A/D converter 15.

Generally, the OFDM transmission/reception apparatus finds a correlationbetween a pre-DFT signal and the pre-DFT signal delayed by a validsymbol length. Then, the OFDM transmission/reception apparatus detects aDFT integration interval by detecting the timing at which thecorrelation result becomes largest. To be more specific, delayer 16delays the reception signal by a valid symbol length, then correlator 17finds a correlation and timing generator 18 detects the timing at whichthe correlation result becomes largest. Guard interval eliminator 19eliminates the guard interval from the reception signal according tothis detection result.

The reception signal stripped of the guard interval is DFT-processed byDFT circuit 20. This results in 4 baseband signals, which are carried by4 carrier. The 4 baseband signals are each subjected to coherentdetection by coherent detectors 21 to 24. In this way, coherent detectedsignals are obtained.

Here, coherent detectors 21 to 24 are explained using FIG. 4. FIG. 4 isa block diagram showing an outlined configuration of the coherentdetector of the OFDM transmission/reception apparatus. Digitalmultipliers 41 and 42 multiply the DFT-processed signals by pilotsymbols. Conjugate complex number generator 43 generates a conjugatecomplex number for the input signal.

In a general frame format, a pilot symbol, a known reference signal, isadded before a message interval. In a general coherent detection method,a fading variation is detected using a pilot symbol.

In (nT) which is a DFT-processed input signal in a pilot symbol intervalis expressed as In (nT)=P (nT) A (nT)·exp (jΘ(nT)), where P (nT) is apilot symbol, A (nT) is an amplitude variation due to fading and exp(jΘ(nT)) is a phase variation due to fading.

F (nT), which represents a variation due to fading, is expressed asfollows:

$\begin{matrix}\begin{matrix}{{F\mspace{14mu}({nT})} = {{In}\mspace{14mu}{({nT}) \cdot P}\mspace{14mu}({nT})}} \\{{= {{\left\{ {P\mspace{14mu}{({nT}) \cdot A}\mspace{14mu}{({nT}) \cdot \exp}\mspace{14mu}\left( {j\;{\Theta({nT})}} \right)} \right\} \cdot P}\mspace{14mu}({nT})}}\mspace{25mu}} \\{= {P\mspace{14mu}{({nT})^{2} \cdot A}\mspace{14mu}{({nT}) \cdot \exp}\mspace{14mu}\left( {j\;{\Theta({nT})}} \right)}}\end{matrix} & {1◯}\end{matrix}$Here, in a modulation system such as a QPSK modulation system in whichthe amplitude is constant and only the phase contains information, P(nT)²=1. Therefore, expression {circle around (1)} is expressed asfollows:F(nT)=A(nT)·exp (jΘ(nT))Then, digital multiplier 41 obtains signal F (nT) that represents avariation due to fading by multiplying DFT-processed input signal(baseband signal) In (nT) by pilot symbol P (nT) in a pilot symbolinterval.

Then, conjugate complex number generator 43 generates a conjugatecomplex number about F (nT), a signal representing a variation due tofading. In this way, conjugate complex number F (nT)* of F (nT) signalexpressing a variation due to fading is obtained. Conjugate complexnumber generator 43 inverts the polarity of the Q component of the inputsignal and generates a conjugate complex number. Therefore, conjugatecomplex number F (nT)* is expressed in the following expression:F(nT)*=A(nT)·exp (−jΘ(nT))

Then, digital multiplier 42 multiplies the DFT-processed input signal(baseband signal) by the conjugate complex number of the signalrepresenting a variation due to fading. In this way, a coherent detectedsignal is obtained.

Here, suppose the fading variation is sufficiently slow compared to theinterval of pilot symbols and the fading variation is constant betweenpilot symbols. Based on this supposition, coherent detected signalD_(out) (nT) is expressed in the following expression:

$\begin{matrix}\begin{matrix}{{D_{out}\mspace{14mu}({nT})} = {D_{in}\mspace{14mu}{({nT}) \cdot A}\mspace{14mu}{({nT}) \cdot \exp}\mspace{14mu}{\left( {{j\Theta}({nT})} \right) \cdot}}} \\{A\mspace{14mu}{({nT}) \cdot \exp}\mspace{14mu}\left( {- {{j\Theta}({nT})}} \right)} \\{= {D_{in}\mspace{14mu}{({nT}) \cdot A}\mspace{14mu}({nT})^{2}}}\end{matrix} & {2◯}\end{matrix}$

In expression {circle around (2)}, A (nT)² is the component with aconstant phase and variable amplitude. Therefore, the phase variation ofcoherent detected signal D_(out) (nT) is only dependent on D_(in) (nT).Therefore, the phase of the reception signal is demodulated by digitalmultiplier 42 multiplying the DFT-processed input signal (basebandsignal) by a conjugate complex number of the signal indicating avariation due to fading. The QPSK modulation system is a modulationsystem with a constant amplitude and variable phase. Therefore, the OFDMtransmission/reception apparatus performs coherent detection bydemodulating the phase of the reception signal.

Furthermore, the OFDM transmission/reception apparatus can alsoeliminate any phase difference between transmission and receptioncarriers and phase variation by frequency offset as well as fadingvariation.

In a modulation system such as a 16QAM modulation system with a variablephase and variable amplitude, the OFDM transmission/reception apparatusdetects a fading variation by dividing an input signal in the pilotsymbol interval by a pilot symbol. The OFDM transmission/receptionapparatus then performs coherent detection by dividing the input signalby a signal indicating a fading variation.

The OFDM transmission/reception apparatus can also use a delay detectionsystem as the demodulation system.

The coherent signals detected by coherent detectors 21 to 24 are judgedby Deciders 25 to 28. After judgment, the 4 signals are converted to asingle signal by P/S converter 29. In this way, a demodulated signal isobtained.

As shown above, the conventional OFDM transmission/reception apparatusadds a signal with the same waveform as that of the last part of a validsymbol at the start of the valid symbol as a guard interval. Byproviding the guard interval, the OFDM transmission/reception apparatuscan eliminate a delayed signal whose delay time is shorter than theguard interval through DFT processing of the reception system.

The explanation above referred to a case where the number of carriers is4, but the same apparatus configuration can also be used when the numberof carriers is increased to 8, 16, 32, 64, and so on.

However, the conventional OFDM transmission/reception apparatus adds aguard interval with a fixed length for every valid symbol independentlyof the channel quality. Therefore, in the case that the channel qualityis good and the delay time of the delayed signal is short, a guardinterval 2 that is longer than necessary is added, which deterioratesthe transmission efficiency.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide an OFDMtransmission/reception apparatus capable of improving the transmissionefficiency while maintaining the function of adding a guard interval toeliminate a delayed signal.

In order to achieve the objective above, the OFDM transmission/receptionapparatus of the present invention changes the length of a guardinterval added at the start of a valid symbol suitably to maintain theminimum necessary length of the guard interval (that is, the guardinterval of an appropriate length) to eliminate a delayed signal at agiven moment and in a given environment.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the invention will appearmore fully hereinafter from a consideration of the following descriptiontaken in connection with the accompanying drawing wherein one example isillustrated by way of example, in which;

FIG. 1 is a block diagram showing an outlined configuration of atransmission system of a conventional OFDM transmission/receptionapparatus;

FIG. 2 is a block diagram showing an outlined configuration of areception system of the conventional OFDM transmission/receptionapparatus;

FIG. 3 is a schematic diagram of a frame format in an OFDM-based radiocommunication;

FIG. 4 is a block diagram showing an outlined configuration of acoherent detector of an OFDM transmission/reception apparatus;

FIG. 5 is a block diagram showing an outlined configuration of an OFDMtransmission/reception apparatus according to Embodiment 1 of thepresent invention;

FIG. 6 is a block diagram showing an outlined configuration of a guardinterval inserter of the OFDM transmission/reception apparatus accordingto Embodiment 1;

FIG. 7 is a timing chart showing an example of the step of a guardinterval inserter of the OFDM transmission/reception apparatus accordingto Embodiment 1 adding a guard interval;

FIG. 8 is a block diagram showing an outlined configuration of anoptimal guard interval length detector of the OFDMtransmission/reception apparatus according to Embodiment 1;

FIG. 9 is a block diagram showing an outlined configuration of anoptimal guard interval length detector of an OFDM transmission/receptionapparatus according to Embodiment 2 of the present invention;

FIG. 10 is a block diagram showing an outlined configuration of anoptimal guard interval length detector of an OFDM transmission/receptionapparatus according to Embodiment 3 of the present invention;

FIG. 11 is a block diagram showing an outlined configuration of an OFDMtransmission/reception apparatus according to Embodiment 4 of thepresent invention;

FIG. 12 is a block diagram showing an outlined configuration of anoptimal guard interval length detector of the OFDMtransmission/reception apparatus according to Embodiment 4;

FIG. 13 is a block diagram showing an outlined configuration of an OFDMtransmission/reception apparatus according to Embodiment 5 of thepresent invention;

FIG. 14 is a graph showing a theoretical calculation result of anenvelope information calculation approximate expression used in a leveldetector of an OFDM transmission/reception apparatus according toEmbodiment 6 of the present invention;

FIG. 15 is a block diagram showing an outlined configuration of a leveldetector of the OFDM transmission/reception apparatus according toEmbodiment 6;

FIG. 16 is a block diagram showing an outlined configuration of an OFDMtransmission/reception apparatus according to Embodiment 7 of thepresent invention;

FIG. 17 is a block diagram showing an outlined configuration of anoptimal guard interval length detector of an OFDM transmission/receptionapparatus according to Embodiment 8 of the present invention;

FIG. 18 is a block diagram showing an outlined configuration of anoptimal guard interval length detector of an OFDM transmission/receptionapparatus according to Embodiment 9 of the present invention;

FIG. 19 is a block diagram showing an outlined configuration of an OFDMtransmission/reception apparatus according to Embodiment 10 of thepresent invention;

FIG. 20 is a block diagram showing an outlined configuration of an OFDMtransmission/reception apparatus according to Embodiment 11 of thepresent invention;

FIG. 21 is a block diagram showing an outlined configuration of an OFDMtransmission/reception apparatus according to Embodiment 12 of thepresent invention;

FIG. 22 is a block diagram showing an outlined configuration of an OFDMtransmission/reception apparatus according to Embodiment 13 of thepresent invention;

FIG. 23 is a block diagram showing an outlined configuration of a UWdetector of the OFDM transmission/reception apparatus according toEmbodiment 13 of the present invention;

FIG. 24 is a block diagram showing an outlined configuration of a UWdetector of an OFDM transmission/reception apparatus according toEmbodiment 14 of the present invention;

FIG. 25 is a block diagram showing an outlined configuration of an OFDMtransmission/reception apparatus according to Embodiment 15 of thepresent invention;

FIG. 26 is a block diagram showing an outlined configuration of a UWdetector of the OFDM transmission/reception apparatus according toEmbodiment 15 of the present invention;

FIG. 27 is a block diagram showing an outlined configuration of an OFDMtransmission/reception apparatus according to Embodiment 16 of thepresent invention;

FIG. 28 is a block diagram showing an outlined configuration of a UWdetector of the OFDM transmission/reception apparatus according toEmbodiment 16 of the present invention;

FIG. 29 is a block diagram showing an outlined configuration of a UWdetector of an OFDM transmission/reception apparatus according toEmbodiment 17 of the present invention;

FIG. 30 is a block diagram showing an outlined Configuration of an OFDMtransmission/reception apparatus according to Embodiment 18 of thepresent invention;

FIG. 31 is a timing chart showing an example of the step of a guardinterval inserter of the OFDM transmission/reception apparatus accordingto Embodiment 18 of the present invention adding a guard interval; and

FIG. 32 is a timing chart showing an example of the step of a guardinterval eliminator of the OFDM transmission/reception apparatusaccording to Embodiment 18 of the present invention eliminating a guardinterval.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to the attached drawings, the embodiments of thepresent invention are explained in detail below.

Embodiment 1

The OFDM transmission/reception apparatus according to Embodiment 1 ofthe present invention is explained using FIG. 5. FIG. 5 is a blockdiagram showing an outlined configuration of the OFDMtransmission/reception apparatus according to Embodiment 1 of thepresent invention.

S/P converter 101 converts a serial input signal to a plurality ofparallel signals. Switches 102 and 103 switch between two input signalsand outputs either of the two. IDFT circuit 104 performs IDFT processingon the input signals. Guard interval inserter 105 inserts a guardinterval into the input signal for every valid symbol. D/A converter 106performs D/A conversion on the signal with a guard interval inserted. Inthis way, a transmission signal is obtained.

A/D converter 107 performs A/D conversion on a reception signal. Delayer108 delays the input signal by a valid symbol length. Correlator 109despreads the input signal. Timing generator 110 detects the timing ofthe reception signal at which the correlation value becomes largest.Guard interval eliminator 111 eliminates the guard interval inserted forevery valid symbol. DFT circuit 112 performs DFT processing on the inputsignal.

Coherent detectors 113 to 116 perform coherent detection on the inputsignal. Deciders 117 to 120 judge the input signals. Switch 121 selectsa control signal to select the optimal guard length from the signaloutput from Decider 120. Subtractor 122 performs a subtraction betweenthe signal input to Decider 117 and the signal output from Decider 117.Optimal guard interval length detector 123 generates a control signal toselect the optimal guard interval length from the output of subtractor122. P/S converter 124 converts a plurality of parallel input signals toa serial signal.

Then, the operation of the OFDM transmission/reception apparatusaccording to the present embodiment is explained. Here, a case where thenumber of carriers is 4 is explained, for example.

First, the operation of the transmission system is explained. The OFDMtransmission/reception apparatus of the present embodiment adds a signalto detect an optimal guard interval length (hereinafter referred to as“optimal guard interval length detection signal”) to a carrier, (here, afirst carrier, for example) and adds a control signal to select anoptimal guard interval length (hereinafter referred to as “optimal guardinterval length selection control signal”) to a carrier (here, a fourthcarrier, for example). The OFDM transmission/reception apparatus of thepresent embodiment can also add an optimal guard interval lengthdetection signal and optimal guard interval length selection controlsignal to a same carrier.

A plurality of symbols contained in the optimal guard interval lengthdetection signal indicates different-guard interval lengths. Here,suppose a symbol period consists of 8 symbols and an optimal guardinterval length detection signal consists of 4 symbols. Therefore, inthe case that the decision error of the first symbol is large, the firstsymbol of the optimal guard interval length detection signal indicatesthat a guard interval length of “symbol period/2” is required. Likewise,the second symbol indicates that a guard interval length of “3×symbolperiod/4” is required; third symbol, a guard interval length of “symbolperiod/8”; and fourth symbol, a guard interval lengths of “symbolperiod/8.”

A demodulated signal input to the transmission system is S/P-convertedby S/P converter 101. This results in 4 demodulated signals, which aretransmitted by a first carrier, second carrier, third carrier and fourthcarrier, respectively.

Switch 102 switches between the demodulated signal transmitted by thefirst carrier and the optimal guard interval length detection signal andoutputs either one to IDFT circuit 104. Furthermore switch 103 switchesbetween the demodulated signal transmitted by the fourth carrier and theoptimal guard interval length selection control signal and outputseither one to IDFT circuit 104.

IDFT circuit 104 performs IDFT processing on the modulated signalsoutput from S/P converter 101, which are transmitted by the second andthird carrier. Furthermore, IDFT circuit 104 performs IDFT processing onthe signals output from switches 102 and 103.

Then, guard interval inserter 105 adds a guard interval into theIDFT-processed signal. An OFDM transmission/reception apparatusgenerally uses a frame format as shown in the frame format schematicdiagram in FIG. 3. That is, in the frame format generally used in anOFDM transmission/reception apparatus, the signal having the samewaveform as that at the last part of a valid symbol is added at thestart of the valid symbol as a guard interval. The OFDMtransmission/reception apparatus can eliminate a delayed signal whosedelay time is shorter than the length of this guard interval through DFTprocessing of the reception system.

The signal with a guard interval added is converted to an analog signalby D/A converter 106 and becomes a transmission signal.

Here, guard interval inserter 105 is explained using FIG. 6. FIG. 6 is ablock diagram showing an outlined configuration of the guard intervalinserter of the OFDM transmission/reception apparatus of the presentembodiment.

S/P converter 201 performs S/P conversion on the optimal guard intervallength selection control signal, which is the output of Decider 120.Switches 202 to 204 switch between two input signals and outputs eitherone. Logical product calculator 205 performs a logical productcalculation of the two input signals. P/S converter 206 converts aplurality of parallel input signals to a serial signal.

The optimal guard interval length selection control signal is split intothree control signals for switches 202, 203 and 204 by S/P converter201.

To switch 202, window signal 1 with a high-level interval of “symbolperiod/2” and window signal 2 with a high-level interval of “3×symbolperiod/8” are input. On the other hand, to switch 203, the output signalof switch 202 and window signal 3 with a high-level interval of “symbolperiod/4” are input. To switch 204, the output signal of switch 203 andwindow signal 4 with a high-level interval of “symbol period/8” areinput. In this way, the number of window signals is the same as thenumber of symbols of the optimal guard interval length detection signal.

Then, switches 202 to 204 select window signals as shown in the tablebelow.

Output of switch 204 Control signal Control signal Control signal(window signal to switch 202 to switch 203 to switch 204 selected) 0 0 0Window signal 1 [symbol period/2] 1 0 0 Window signal 2 [3 × symbolperiod/8] 1 1 0 Window signal 3 [symbol period/4] 1 1 1 Window signal 4[symbol period/8]

As shown above, the OFDM transmission/reception apparatus of the presentembodiment selects window signals using the decision error of eachsymbol of the optimal guard interval length detection signal. Then, theOFDM transmission/reception apparatus of the present embodiment candecide the guard interval length by calculating a logical product of theselected window signal (output of switch 204) and a valid symbol.

Logical product calculator 205 calculates the logical product of theoutput of switch 204 and the output of IDFT circuit 104. In this way,the OFDM transmission/reception apparatus of the present embodiment canextract part of a valid symbol, and thus can generate a guard interval.

Then, P/S converter 206 performs P/S conversion of the guard intervalsignal, which is the output of logical product calculator 205, and theoutput signal of IDFT circuit 104. In this way, an IDFT signal with aguard interval inserted is obtained.

Here, the step of guard interval inserter 105 generating guard intervalsis explained using FIG. 7. FIG. 7 is a timing chart showing an exampleof the step of the guard interval inserter of the OFDMtransmission/reception apparatus of the present embodiment generatingguard intervals.

The signal shown at A represents a valid symbol. Likewise, B representswindow signal 1; C, window signal 2; D, window signal 3; E, windowsignal 4; F, guard interval signal; G, valid signal delayed by 1 symbol;and H, IDFT signal after addition of a guard interval.

The OFDM transmission/reception apparatus of the present embodimentselects one of window signals with different high-level interval lengthsand finds the logical product of the window signal and the valid symbol.In this way, the OFDM transmission/reception apparatus of the presentembodiment can set the same number of guard interval lengths as thenumber of window signals.

For example, the guard interval signal shown at F is generated asfollows. That is, the OFDM transmission/reception apparatus of thepresent embodiment calculates the logical product of the valid symbolshown at A and window signal 3 shown at D to extract the symbol with thesame length as the high-level interval of window signal 3 shown at Dfrom the last part of valid symbol A. In this way the guard intervalsignal shown at F is generated.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment selects a window signal with a different high-level intervallength every time an optimal guard interval length detection signalinserted into a transmission signal is received. Then, the OFDMtransmission/reception apparatus of the present embodiment calculatesthe logical product of the selected window signal and valid symbol togenerate a guard interval according to the channel quality. In this way,the OFDM transmission/reception apparatus of the present embodiment canexpand/contract a guard interval length according to the channelquality.

Then, the operation of the reception system of the OFDMtransmission/reception apparatus of the present embodiment is explainedusing FIG. 5.

The reception signal input to the reception system is converted to adigital signal by A/D converter 107.

The OFDM transmission/reception apparatus of the present embodimentcalculates a correlation between a pre-DFT signal and the pre-DFT signaldelayed by a valid symbol length. The OFDM transmission/receptionapparatus of the present embodiment detects the integration interval ofDFT by detecting the timing at which the correlation result becomeslargest. More specifically, delayer 108 delays the reception signal bythe valid symbol length, then correlator 109 calculates a correlationand timing generator 110 detects the timing at which the correlationresult becomes largest. Guard interval eliminator 111 eliminates theguard interval from the reception signal according to this detectionresult.

The reception signal stripped of the guard interval is DFT-processed byDFT circuit 112. This results in 4 baseband signals, which are carriedby 4 carriers. The 4 baseband signals are subjected to coherentdetection by coherent detectors 113 to 116, respectively. In this way,coherent detected signals are obtained. Here, a delay detection systemcan also be used as the demodulation system.

The coherent detected signals obtained by coherent detectors 113 to 116are judged by Deciders 117 to 120. The 4 signals judged by Deciders 117to 120 are converted to a serial signal by P/S converter 124. In thisway, a demodulated signal is obtained.

On the other hand, switch 121 selects only the optimal guard intervallength selection control signal from the output signal of Decider 120and outputs it to guard interval inserter 105.

Subtractor 122 carries out a subtraction between the signal input toDecider 117 and the signal output from Decider 117 to calculate thedecision error. Here, this decision error is determined as the channelquality. Optimal guard interval length detector 123 generates an optimalguard interval length selection control signal using the decision errorcalculated by subtractor 122, that is, channel quality information.

Here, the optimal guard interval length detector of the OFDMtransmission/reception apparatus of the present embodiment is explainedusing FIG. 8. FIG. 8 is a block diagram showing an outlinedconfiguration of the optimal guard interval length detector of the OFDMtransmission/reception apparatus of the present embodiment.

The decision error input to optimal guard interval length detector 123is selectively output by switch 401 and S/P-converted by S/P converter402.

Subtractor 403 calculates a subtraction between the first output of S/Pconverter 402 and the second output of S/P converter 402. Likewise,subtractor 404 calculates a subtraction between the first output and thethird output, and subtractor 405 calculates a subtraction between thefirst output and the fourth output.

In this way, optimal guard interval length detector 123 finds adifference between the decision error of the symbol indicating thelongest guard interval length (here, decision error of the symbolindicating “symbol period/2” which is the first output of S/P converter402) and the decision error of another symbol.

Furthermore, subtractor 406 calculates a subtraction between the outputof subtractor 403 and a threshold. Likewise, subtractor 407 calculates asubtraction between the output of subtractor 404 and the threshold, andsubtractor 408 calculates a subtraction between the output of subtractor405 and the threshold. Then, Deciders 409 to 411 judge the outputs ofsubtractors 406 to 408.

As shown above, decision errors are compared with thresholds. Then,guard interval inserter 105 inserts the guard interval with the shortestguard interval of the guard intervals indicated by the symbols producingdecision errors less than the threshold into the transmission signal.

That is, Deciders 409 to 411 output a high level (1, for example) in thecase that the decision error is not less than the threshold (that is,the channel quality is good) and output a low level (0, for example) inthe case that the decision error is less than the threshold. Then, theoutputs of Deciders 409 to 411 are P/S-converted by P/S converter 412 tobecome an optimal guard interval length selection control signal. Thisoptimal guard interval length selection control signal is included inthe transmission signal and transmitted.

The received optimal guard interval length selection control signal isS/P-converted to control signals for switches 202 to 204 by S/Pconverter 201. That is, the outputs of Deciders 409 to 411 becomecontrol signals for switches 202 to 204. The guard interval insertionprocedure hereafter is as already described.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment detects the reception condition of each symbol of an optimalguard interval length detection signal and decides up to which of the 4symbols have been received. This allows the OFDM transmission/receptionapparatus of the present embodiment to detect the minimum necessaryguard interval length to eliminate delayed signals, making it possibleto change the guard interval to an appropriate length suitably.

The length of a guard interval to be added to each symbol should belonger than the maximum delay time of a delayed signal assumed in anenvironment in which the OFDM transmission/reception apparatus of thepresent embodiment is used. However, such a delayed signal with themaximum delay time assumed does not always exist. Therefore, asdescribed above, the OFDM transmission/reception apparatus of thepresent embodiment can improve the transmission efficiency withoutdeteriorating the error rate characteristic by adaptively changing thelength of guard intervals to prevent any guard interval that is longerthan required from being provided.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment can improve the transmission efficiency by adaptivelychanging the length of guard intervals.

The present embodiment is explained assuming that the number of carriersis 4, but the number of carriers can be increased to 8, 16, 32, and 64,and so on with an apparatus configuration similar to the configurationabove.

In the present embodiment, the types of window signals are not limitedto the 4 types above, but any number of window signals with apredetermined high-level intervals can be set.

Furthermore, in the present embodiment, FFT (Fast Fourier Transform) andIFFT (Inverse Fast Fourier Transform) can also be used instead of DFTand IDFT. Similar effects can be achieved in such cases, too.

Embodiment 2

The OFDM transmission/reception apparatus of Embodiment 2 of the presentinvention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 1 and uses symbols ofoptimal guard interval length detection signals of a plurality ofbursts.

The optimal guard interval length detector of the OFDMtransmission/reception apparatus of the present embodiment is explainedbelow using FIG. 9. FIG. 9 is a block diagram showing an outlinedconfiguration of the optimal guard interval length detector of the OFDMtransmission/reception apparatus of Embodiment 2 of the presentinvention. The parts with the same configuration as that of Embodiment 1are assigned the same numbers and their detailed explanations areomitted.

Averagers 501 to 503 perform averaging on the outputs of subtractors 403to 405.

In this way, the optimal guard interval length detector performsaveraging using symbols of optimal guard interval length detectionsignals of a plurality of bursts. Therefore, the OFDMtransmission/reception apparatus of the present embodiment can achievehigher accuracy of an optimal guard interval length selection controlsignal than that of the OFDM transmission/reception apparatus ofEmbodiment 1.

Embodiment 3

The OFDM transmission/reception apparatus of Embodiment 3 of the presentinvention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 1 and inserts symbols foroptimal guard interval length detection signals into a plurality ofcarriers.

The optimal guard interval length detector of the OFDMtransmission/reception apparatus of the present embodiment is explainedbelow using FIG. 10. FIG. 10 is a block diagram showing an outlinedconfiguration of the optimal guard interval length detector of the OFDMtransmission/reception apparatus of Embodiment 3 of the presentinvention. The parts with the same configuration as that of Embodiment 1are assigned the same numbers and their detailed explanations areomitted.

The transmission system of the OFDM transmission/reception apparatus ofthe present embodiment has switch 601. With switch 601, an optimal guardinterval length detection signal is inserted into not only carrier 1 butalso carrier 2.

Furthermore, the reception system of the OFDM transmission/receptionapparatus of the present embodiment also has subtractor 602 and Averager603. Subtractor 602 calculates the decision error of carrier 2. Averager603 calculates the average of decision errors of carrier 1 and carrier2. Averager 603 outputs the average value of the decision errors tooptimal guard interval length detector 123.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment inserts an optimal guard interval length detection signalinto a plurality of carriers and generates an optimal guard intervallength selection control signal using the average value of decisionerrors. This allows the OFDM transmission/reception apparatus of thepresent embodiment to achieve higher accuracy of optimal guard intervallength selection control signal than that of the OFDMtransmission/reception apparatuses of Embodiments 1 and 2.

Embodiment 4

The OFDM transmission/reception apparatus of Embodiment 4 of the presentinvention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 3, but does not averageoptimal guard interval length detection signals before they are input tothe optimal guard interval length detector and performs a logicalproduct calculation on the optimal guard interval length detectionsignals using the optimal guard interval length detector.

The optimal guard interval length detector of the OFDMtransmission/reception apparatus of the present embodiment is explainedbelow using FIG. 11 and FIG. 12. FIG. 11 is a block diagram showing anoutlined configuration of the OFDM transmission/reception apparatus ofEmbodiment 4 of the present invention. FIG. 12 is a block diagramshowing an outlined configuration of the optimal guard interval lengthdetector of the OFDM transmission/reception apparatus of Embodiment 4 ofthe present invention. The parts with the same configuration as that ofEmbodiments 1 and 3 are assigned the same numbers and their detailedexplanations are omitted.

As shown in FIG. 11, the OFDM transmission/reception apparatus of thepresent embodiment does not average the decision errors of carrier 1 andcarrier 2, and inputs the decision errors of carrier 1 and carrier 2 tooptimal guard interval length detector 701.

Then, as shown in FIG. 12, logical product calculators 801 to 803perform a logical product calculation on the outputs of Deciders 409 to411 for carrier 1 and the outputs of Deciders 409 to 411 for carrier 2.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment can achieve higher accuracy of an optimal guard intervallength selection control signal by selecting a guard interval lengthwhose difference between decision errors falls below a threshold as theoptimal guard interval length in all carriers into which symbols foroptimal guard interval detection have been inserted.

Embodiment 5

The OFDM transmission/reception apparatus of Embodiment 5 of the presentinvention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 3 but does not usecarriers whose reception level falls below a threshold to detect theoptimal guard interval length.

The optimal guard interval length detector of the OFDMtransmission/reception apparatus of the present embodiment is explainedbelow using FIG. 13. FIG. 13 is a block diagram showing an outlinedconfiguration of the OFDM transmission/reception apparatus of Embodiment5 of the present invention. The parts with the same configuration asthat of Embodiments 1 and 3 are assigned the same numbers and theirdetailed explanations are omitted.

Square sum calculator 901 calculates the sum of squares of coherentdetected signals of carrier 1. Square sum calculator 902 calculates thesum of squares of coherent detected signals of carrier 2.

Then, subtractor 903 carries out a subtraction between the output ofsquare sum calculator 901 and a threshold and Decider 905 decides whichis larger/smaller based on the subtraction result. Subtractor 904carries out a subtraction between the output of square sum calculator902 and the threshold and Decider 906 decides which is larger/smallerbased on the subtraction result.

Switches 907 and 908 are controlled by Deciders 905 and 906,respectively. In the case that the reception levels of their respectivecarriers (that is, the outputs of square sum calculators 901 and 902)are less than the threshold, switches 907 and 908 do not output thedecision errors of the carriers to Averager 603.

As shown above, based on the fact that the reception levels vary foreach carrier in a frequency selection fading environment, the OFDMtransmission/reception apparatus of the present embodiment assumes thatthe carrier whose reception level falls below the threshold does nothave good channel quality and does not use the carrier whose receptionlevel falls below the threshold for detection of the optimal guardinterval length. This allows the OFDM transmission/reception apparatusof the present embodiment to achieve higher accuracy of optimal guardinterval length selection control signals.

Embodiment 6

The OFDM transmission/reception apparatus of Embodiment 6 of the presentinvention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 5 and has a leveldetector with a simple configuration instead of the square sumcalculator.

The present embodiment describes a case where the input signal is aQPSK-modulated signal and the known reference signal is a pilot symbol.

The level detector of the present embodiment finds envelope informationfrom the absolute values of the I component and Q component byapproximate calculation and detects the reception level.

Envelope information Z can be obtained from Z=√(|I|²+|Q|²). However,obtaining a square sum from √(|I|²+|Q|²) requires a relatively greatamount of calculation. Therefore, to reduce the amount of calculation toa minimum, it is possible to find a square sum by approximatecalculation using approximate expression Z=|I|+|Q|. However, in the casethat this approximate expression is used the square sum becomes amaximum (when phase is 45°) of 1.414 times the value calculate by√(|I|²+|Q|²) That is, an error of a maximum of approximately 41% willresult, deteriorating the error rate characteristic.

Therefore, the OFDM transmission/reception apparatus of the presentembodiment uses an approximate expression using a simple multiplicationby a bit shift. That is, the OFDM transmission/reception apparatus ofthe present embodiment uses Z=|I|+0.375×|Q| when |I|>|Q| andZ=|Q|+0.375×|I| when |Q|>|I| as an approximate expression.

FIG. 14 is a graph showing the result of a theoretical calculation ofthe relationship between phase θ and estimated radius (that isamplitude) when |I|>|Q| (that is, range of 0≦θ≦45°). From this graph, itis clearly seen that using the above approximate expression makes itpossible to obtain envelope information with an error within 7% of thecase obtained with a square sum.

Then, the level detector of the OFDM transmission/reception apparatus ofthe present embodiment obtains envelope information using the aboveapproximate expression and detects the reception level.

The level detector of the OFDM transmission/reception apparatus of thepresent embodiment is explained using FIG. 15. FIG. 15 is a blockdiagram showing an outlined configuration of the level detector of theOFDM transmission/reception apparatus of Embodiment 6.

The I component and Q component of one carrier after coherent detectionprocessing are input to absolute value detectors 1101 and 1102. Absolutevalue detectors 1101 and 1102 calculate the absolute values of inputsignals and outputs the absolute values to subtractor 1105 and adder1110. The I component or Q component is selected by switches 1103 and1104. The subtraction result from subtractor 1105 is judged by Decider1106. The decision results are used to control switches 1103 and 1104.

2-bit shifter 1107 and 3-bit shifter 1108 shift the output of switch1104 by 2 bits and 3 bits, respectively. The outputs of 2-bit shifter1107 and 3-bit shifter 1108 are added up by adder 1109. This allowsmultiplication processing of 0.375 in the above approximate expressionto be performed. Adder 1110 adds up the output of switch 1103 and theoutput of adder 1109 and outputs envelope information.

Then, the operation of the level detector of the OFDMtransmission/reception apparatus of the present embodiment is explained.

The absolute values of the I component and Q component are calculated byabsolute value detectors 1101 and 1102, respectively. In this way,absolute values |I| and |Q| are obtained.

Then, subtractor 1105 performs a subtraction between the outputs ofabsolute value detectors 1101 and 1102 (|I| and |Q|). Decider 1106decides which is larger/smaller based on the subtraction result. Theoutputs of absolute value detectors 1101 and 1102 (|I| and |Q|) areselected by switches 1103 and 1104, respectively and output. Switches1103 and 1104 select the signal to be output according to the decisionresult of Decider 1106.

When the output of Decider 1106 indicates |I|>|Q|, switch 1103 outputs|I|. When the output of Decider 1106 indicates |Q|>|I|switch 1103outputs |Q|. Furthermore, when the output of Decider 1106 indicates|I|>|Q|, switch 1104 outputs |Q|. When the output of Decider 1106indicates |Q|>|I|, switch 1104 outputs |I|. In short, switch 1103outputs |I| or |Q|, whichever is greater and switch 1104 outputs |I| or|Q|, whichever is smaller.

The smaller one of |I| or |Q| output from switch 1104 is shifted by 2bits and 3 bits by 2-bit shifter 1107 and 3-bit shifter 1108,respectively.

Since a 1-bit shift reduces the amplitude to half, the amplitude becomes0.25 times with a 2-bit shift and 0.125 times with a 3-bit shift.Therefore, the amplitude of the output signal of 2-bit shifter 1107becomes 0.25 times the amplitude of the output signal of switch 1104.The amplitude of the output signal of 3-bit shifter 1108 becomes 0.125times the amplitude of the output signal of switch 1104.

Then, adder 1109 adds up the output signal of 2-bit shifter 1107(0.25×|I| or 0.25×|Q|) and the output signal of 3-bit shifter 1108(0.125×|I| or 0.125×|Q|). Therefore, the output signal of adder 1109becomes 0.375×|I| or 0.375×|Q|.

Then, adder 1110 adds up the output signal of switch 1103 (|I| or |Q|)and the output signal of adder 1109 (0.375×|I| or 0.375×|Q|). From this,envelope information Z is obtained from the approximate expressionabove.

As shown above, the level detector of the OFDM transmission/receptionapparatus of the present embodiment detects the reception level byfinding an envelope, and thus it does not perform a multiplicationunlike the square sum calculator of Embodiment 5. Therefore, the OFDMtransmission/reception apparatus of the present embodiment has a simplerconfiguration, making it possible to reduce the necessary amount ofcalculation.

Moreover, the OFDM transmission/reception apparatus of the presentembodiment can further reduce the necessary amount of calculation incalculating an envelope by using an approximate expression onlyconsisting of simple multiplications and additions, which can beimplemented by bit shifts on the circuit.

The present embodiment describes the case where the input signal is aQPSK-modulated signal. However, the above apparatus configuration isalso applicable to any other cases where other modulation systems areused so long as the input signal can be-separated into the I componentand Q component.

Embodiment 7

The OFDM transmission/reception apparatus of Embodiment 7 of the presentinvention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 1 and uses a known symbolas the optimal guard interval length detection signal.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 16. FIG. 16 is a block diagram showing anoutlined configuration of the OFDM transmission/reception apparatus ofEmbodiment 7 of the present invention. The parts with the sameconfiguration as that of Embodiment 1 are assigned the same numbers andtheir detailed explanations are omitted.

As shown in FIG. 16, the OFDM transmission/reception apparatus of thepresent embodiment uses a known symbol as the optimal guard intervallength detection signal. Subtractor 1201 performs a subtraction betweenthe carrier 1 signal prior to its input to Decider 117 and a knownsymbol and outputs the difference between these two to optimal guardinterval length detector 123.

Thus, the OFDM transmission/reception apparatus of the presentembodiment uses a known symbol as the optimal guard interval lengthdetection signal, therefore can improve the accuracy of the optimalguard interval length selection control signal.

Embodiment 8

The OFDM transmission/reception apparatus of Embodiment 8 of the presentinvention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 1 and uses a variablethreshold to be used in the optimal guard interval length detector.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 17. FIG. 17 is a block diagram showing anoutlined configuration of the optimal guard interval length detector ofthe OFDM transmission/reception apparatus of Embodiment 8 of the presentinvention. The parts with the same configuration as that of Embodiment 1are assigned the same numbers and their detailed explanations areomitted.

Switch 1301 selects an input decision error and outputs it to S/Pconverter 402 and Averager 1302. The previous burst channel qualityinformation is stored in memory 1304 by the switching of switch 1303.

The channel quality information stored in memory 1304 is subjected to asubtraction with threshold A by subtractor 1305. Decider 1306 decideswhich is larger/smaller based on the subtraction result. Switch 1307 iscontrolled by the decision result of Decider 1306 and outputs eitherthreshold B or threshold C. Here, suppose threshold B>threshold C.

As shown above, considering the possibility that if, for example, thechannel quality is bad, variations in the decision error will increaseand a longer guard interval length than the optimal guard intervallength will be selected, the OFDM transmission/reception apparatus ofthe present embodiment can improve the accuracy of the optimal guardinterval length selection control signal by changing the threshold to agreater value in the case that the decision error information stored inmemory exceeds the threshold.

Embodiment 9

The OFDM transmission/reception apparatus of Embodiment 9 of the presentinvention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 1, and selects a guardinterval length in which a difference between decision errors fallsbelow a threshold in a plurality of bursts as the optimal guard intervallength.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 18. FIG. 18 is a block diagram showing anoutlined configuration of the optimal guard interval length detector ofthe OFDM transmission/reception apparatus of Embodiment 9 of the presentinvention. The parts with the same configuration as that of Embodiment 1are assigned the same numbers and their detailed explanations areomitted.

In FIG. 18, counters 1401 to 1403 output the outputs of Deciders 409 to411 for every a plurality of bursts. Subtractors 1404 to 1406 perform asubtraction between the outputs of counters 1401 to 1403 and athreshold. Deciders 1407 to 1409 decide which is larger/smaller based onthe subtraction results.

Thus, the OFDM transmission/reception apparatus of the presentembodiment selects a guard interval length in which a difference betweendecision errors falls below a threshold in a plurality of bursts as theoptimal guard interval length, thus improving the accuracy of theoptimal guard interval length selection control signal.

Embodiment 10

The OFDM transmission/reception apparatus of Embodiment 10 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 1, and sets the maximumguard interval length in the case that the channel quality of thecarrier into which an optimal guard interval length selection controlsignal is inserted falls below a threshold.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 19. FIG. 19 is a block diagram showing anoutlined configuration of the OFDM transmission/reception apparatus ofEmbodiment 9 of the present invention. The parts with the sameconfiguration as that of Embodiment 1 are assigned the same numbers andtheir detailed explanations are omitted.

In FIG. 19, subtractor 1501 performs a subtraction on the carrier intowhich an optimal guard interval length selection control signal isinserted (here carrier 4) between the signal before being input toDecider 120 and the output signal from Decider 120. That is, subtractor1501 calculates a decision error of carrier 4.

Then, the decision error of carrier 4 is averaged by Averager 1502.Subtractor 1503 performs a subtraction between the averaged decisionerror and a threshold. Decider 1504 decides which is larger/smallerbased on the subtraction result. Switch 1505 is controlled by thedecision result of Decider 1504 and selects either the output of optimalguard interval length detector 123 or the control signal indicating themaximum guard interval and outputs it to switch 103.

Thus, the OFDM transmission/reception apparatus of the presentembodiment sets the maximum guard interval length in the case that thechannel quality of the carrier into which an optimal guard intervallength selection control signal is inserted falls below a threshold.Thus, in the case that an error exists in the optimal guard intervallength selection control signal after demodulation, the OFDMtransmission/reception apparatus of the present embodiment can preventthe optimal guard interval length from failing to be set, and thusprevent the error rate characteristic from deteriorating.

By the way, the OFDM transmission/reception apparatus of the presentembodiment can also achieve the same effect as that described above inthe case that it performs error detection on the carrier into which theoptimal guard interval length selection control signal is inserted andset the maximum guard interval length when an error is detected.

Embodiment 11

The OFDM transmission/reception apparatus of Embodiment 11 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 1, and eliminates theneed for transmission/reception of a control signal about the guardinterval length by transmitting a signal using the optimal guardinterval length detected from a reception signal in both radio stationscarrying out a radio communication.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 20. FIG. 20 is a block diagram showing anoutlined configuration of the OFDM transmission/reception apparatus ofEmbodiment 11 of the present invention. The parts with the sameconfiguration as that of Embodiment 1 are assigned the same numbers andtheir detailed explanations are omitted.

The present embodiment describes a case where a radio communication iscarried out using a OFDM/TDD system.

In FIG. 20, optimal guard interval length detector 1601 outputs acontrol signal about the guard interval length to guard intervalinserter 105.

In a TDD system, a same frequency is used for the uplink and downlink,and therefore the channel information is identical for the uplink anddownlink. Therefore, the OFDM transmission/reception apparatus of thepresent embodiment can eliminate the need for transmission/reception ofthe optimal guard interval length selection control signal bytransmitting a signal using the optimal guard interval length detectedfrom the reception signal in both radio stations-carrying out a radiocommunication.

In the case that the OFDM transmission/reception apparatus of thepresent embodiment uses the optimal guard interval length selectioncontrol signal, the OFDM transmission/reception apparatus of the presentembodiment can also detect the optimal guard interval length even if anerror exists in the optimal guard interval length selection controlsignal after demodulation. Thus, the OFDM transmission/receptionapparatus of the present embodiment can prevent the error ratecharacteristic of the control signal from deteriorating.

Embodiment 12

The OFDM transmission/reception apparatus of Embodiment 12 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 11, and detects theoptimal guard interval length using a control channel signal.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 21. FIG. 21 is a block diagram showing anoutlined configuration of the OFDM transmission/reception apparatus ofEmbodiment 12 of the present invention. The parts with the sameconfiguration as that of Embodiment 11 are assigned the same numbers andtheir detailed explanations are omitted.

In FIG. 21, switches 1701 and 1702 are controlled by a control channelsignal whose error is corrected more strongly than the user channelsignal. That is, the control channel signal controls the timing ofinserting an optimal guard interval length detection signal in thetransmission system and the timing of extracting an optimal guardinterval length detection signal in the reception system.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment detects the guard interval length using a control channelsignal, and therefore can reduce the probability that an error willoccur in the control signal that the optimal guard interval lengthdetector outputs.

Embodiment 13

The OFDM transmission/reception apparatus of Embodiment 13 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 1, and controls theselection of the guard interval length to be added to a valid symbolusing a known signal (Unique Word; hereinafter referred to as “UW”) toacquire synchronization of a plurality of frames.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 22 and FIG. 23. FIG. 22 is a block diagramshowing an outlined configuration of the OFDM transmission/receptionapparatus of Embodiment 13 of the present invention. FIG. 23 is a blockdiagram showing an outlined configuration of a UW detector of the OFDMtransmission/reception apparatus of Embodiment 13 of the presentinvention. The parts with the same configuration as that of Embodiment 1are assigned the same numbers and their detailed explanations areomitted.

In FIG. 22, the output of optimal guard interval length detector 123controls switch 1801. One of UW1 to UW4 is selected by switch 1801according to this control and output to switch 103.

Here, in the case that the decision error of UW1 is bad, UW1 indicatesthat a guard interval length of “symbol period/2” is necessary.Likewise, UW2 indicates that a guard interval length of “3× symbolperiod/8” is necessary; UW3, guard interval length of “symbol period/4”;and UW4, guard interval length of “symbol period/8.”

UW detector 1802 detects UW in the demodulated reception signal. Thedetected UW is selectively output to guard interval inserter 105 byswitch 1803.

Generally, in frame synchronization acquisition using UW, an exclusiveOR calculation between UW and a demodulated signal is performed. In thecase that the accumulated value of the results of the exclusive ORcalculations exceeds a threshold, it is determined that framesynchronization has been acquired. Here, even if errors exist in the UWafter demodulation in the case that the number of those errors is withinthe range that the accumulated value of the exclusive OR calculationresults exceeds a threshold, frame synchronization is correctlyacquired.

In FIG. 23, exclusive OR calculators 1901 to 1904 perform exclusive ORcalculations between the demodulated signal input to UW detector 1802and each of UW1 to UW4. Subtractors 1905 to 1908 perform subtractionsbetween the outputs of exclusive OR calculators 1901 to 1904 and athreshold.

Deciders 1909 to 1912 judge the outputs of subtractors 1905 to 1908.Then P/S converter 1913 converts 4 parallel decision data to singleserial data.

Logical sum calculator 1914 performs a logical sum calculation of theoutputs of subtractors 1905 to 1908. Switch 1915 is controlled by theoutput of logical sum calculator 1914 and selectively outputs the outputof P/S converter 1913.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment acquires frame synchronization using a plurality of UWs andcontrols the selection of a guard interval length according to theinformation on which UW has acquired frame synchronization. This allowsthe OFDM transmission/reception apparatus of the present embodiment toreduce the probability that an error will exist in the guard intervallength selection control signal and thus prevent the error ratecharacteristic from deteriorating.

By the way, the OFDM transmission/reception apparatus of the presentembodiment is especially valid for an OFDM system other than TDD.

Embodiment 14

The OFDM transmission/reception apparatus of Embodiment 14 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 13, and reduces thenumber of UWs used by the UW detector.

The UW detector of the OFDM transmission/reception apparatus of thepresent embodiment is explained below using FIG. 24. FIG. 24 is a blockdiagram showing an outlined configuration of the UW detector of the OFDMtransmission/reception apparatus of Embodiment 14 of the presentinvention. The parts with the same configuration as that of Embodiment13 are assigned the same numbers and their detailed explanations areomitted.

In FIG. 24, exclusive OR calculator 2001 performs an exclusive ORcalculation of the demodulated signal and UW1. Exclusive OR calculator2001 outputs the result of the exclusive OR calculation of thedemodulated signal and UW1 to subtractor 2003. Exclusive OR calculator2001 also outputs the result of the exclusive OR calculation of thedemodulated signal and the inverted signal of UW1 to subtractor 2004.Likewise, exclusive OR calculator 2002 performs an exclusive ORcalculation of the demodulated signal and UW2. Exclusive OR calculator2002 outputs the result of the exclusive OR calculation of thedemodulated signal and UW2 to subtractor 2005. Exclusive OR calculator2002 also outputs the result of the exclusive OR calculation of thedemodulated signal and the inverted signal of UW2 to subtractor 2006.

Here, in the case that the decision error of UW1 is large, UW1 indicatesthat a guard interval length of “symbol period/2” is necessary.Likewise, the inverted signal of UW1 indicates a guard interval lengthof “3×symbol period/8” is necessary; UW2, a guard interval length of“symbol period/4”; and the inverted signal of UW2, a guard intervallength of “symbol period/8.”

As shown above, the OFDM transmission/reception apparatus of the presentembodiment can reduce to half the number of exclusive OR calculationscarried out by the UW detector by using the inverted signal of UW. Thisreduces the amount of calculation and simplifies the circuitconfiguration.

Embodiment 15

The OFDM transmission/reception apparatus of Embodiment 15 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 13, and carries out UWdetection using a demodulated signal prior to decision processing.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 25 and FIG. 26. FIG. 25 is a block diagramshowing an outlined configuration of the OFDM transmission/receptionapparatus of Embodiment 15 of the present invention. FIG. 26 is a blockdiagram showing an outlined configuration of a UW detector of the OFDMtransmission/reception apparatus of Embodiment 15 of the presentinvention. The parts with the same configuration as that of Embodiment13 are assigned the same numbers and their detailed explanations areomitted.

As shown in FIG. 25, synchronization detection signals before beinginput to Deciders 117 to 120 are input to UW detector 2101.

Furthermore, as shown in FIG. 26, the demodulated signals input to UWdetector 2101 are converted to a serial signal by P/S converter 2201.Multipliers 2202 to 2205 multiply the output signal of P/S converter2201 by UW1 to UW4, respectively.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment acquires frame synchronization using coherent detectedsignals prior to decision processing, and thus can reduce theprobability that an error will occur in a control signal.

Embodiment 16

The OFDM transmission/reception apparatus of Embodiment 16 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 13, and uses a variablethreshold in UW detectors according to decision errors.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 27 and FIG. 28. FIG. 27 is a block diagramshowing an outlined configuration of the OFDM transmission/receptionapparatus of Embodiment 16 of the present invention. FIG. 28 is a blockdiagram showing an outlined configuration of the UW detector of the OFDMtransmission/reception apparatus of Embodiment 16 of the presentinvention. The parts with the same configuration as that of Embodiment13 are assigned the same numbers and their detailed explanations areomitted.

As shown in FIG. 27, the decision error of carrier 1, which is theoutput of subtractor 122, together with a demodulated signal is input toUW detector 2301. The decision error need not always be from carrier 1.

Furthermore, as shown in FIG. 28, subtractor 2401 performs a subtractionbetween the decision error input to UW detector 2301 and threshold A.Then, Decider 2402 decides which is larger/smaller based on thesubtraction result. Switch 2403 is controlled by the output of Decider2402. Switch 2403 outputs threshold B in the case that the decisionerror is not less than the threshold and outputs threshold C in the casethat the decision error is less than the threshold. Here, supposethreshold B>threshold C.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment uses a variable threshold used to acquire framesynchronization in the UW detectors according to the channel quality.That is, the OFDM transmission/reception apparatus of the presentembodiment decreases the threshold when the channel quality is bad. Thisallows the OFDM transmission/reception apparatus of the presentembodiment to improve the accuracy of frame synchronization acquisition.Moreover, in the case that the channel quality is bad, the OFDMtransmission/reception apparatus of the present embodiment can alsoimprove the accuracy of frame synchronization acquisition by reducingthe threshold using the channel quality information of the previousburst (for example, decision error).

Embodiment 17

The OFDM transmission/reception apparatus of Embodiment 17 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 13, and in the case thatthe decision error in the UW detector exceeds a threshold, it controlsin such a way that the guard interval length is maintained.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 29. FIG. 29 is a block diagram showing anoutlined configuration of the UW detector of the OFDMtransmission/reception apparatus of Embodiment 17 of the presentinvention. The parts with the same configuration as that of Embodiments13 and 16 are assigned the same numbers and their detailed explanationsare omitted.

In FIG. 29, switch 2501 is controlled by the output of Decider 2402.Switch 2501 selects either the output of switch. 1915 or a zero valueand outputs it.

As shown above, in the case that the channel quality is bad in the UWdetector, the OFDM transmission/reception apparatus of the presentembodiment outputs a zero value to maintain the guard interval lengthwithout changing the threshold for acquisition of frame synchronization.Therefore, the OFDM transmission/reception apparatus of the presentembodiment can prevent the error rate characteristic from deterioratingor frame synchronization from failing to be acquired, a situation whichis likely to occur when a threshold is changed with a bad channelquality.

Embodiment 18

The OFDM transmission/reception apparatus of Embodiment 18 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 1, and does not changethe length of a guard interval to be added to a valid symbol indicatingimportant information, but always keeps it constant independently of thechannel quality.

The “guard interval length necessary to eliminate delayed signal”described so far in Embodiments 1 to 17 has been the length apparentlyenough to achieve an error rate in a communication related to user datasuch as a message (hereinafter referred to as “normal information”).However, higher accuracy is required for important information such ascontrol information and retransmission information than normalinformation in terms of the error rate.

Thus, the OFDM transmission/reception apparatus of the presentembodiment distinguishes important information from normal informationin a transmission signal and does not change the length of a guardinterval to be added to the valid symbol indicating importantinformation according to the channel quality, but always keeps itconstant.

The OFDM transmission/reception apparatus of the present embodiment setsthe guard interval length to be added to the valid symbol indicating theimportant information above to a certain length that allows an errorrate lower than the error rate achieved by the guard interval to beadded to the valid symbol indicating normal information.

The OFDM transmission/reception apparatus of the present embodiment isexplained below using FIG. 30 to FIG. 32. FIG. 30 is a block diagramshowing an outlined configuration of the OFDM transmission/receptionapparatus of Embodiment 18 of the present invention. FIG. 31 is a timingchart showing an example of the step of the guard interval inserter ofthe OFDM transmission/reception apparatus of Embodiment 18 of thepresent invention adding a guard interval. FIG. 32 is a timing chartshowing an example of the step of the guard interval eliminator of theOFDM transmission/reception apparatus of Embodiment 18 of the presentinvention eliminating a guard interval. The parts with the sameconfiguration as that of Embodiment 1 are assigned the same numbers andtheir detailed explanations are omitted.

In FIG. 30, a known timing control signal is input to guard intervalinserter 2601. Therefore, guard interval inserter 2601 can distinguish asymbol indicating normal information from another symbol indicatingimportant in formation in the transmission symbol based on this timing.

As described above, guard interval inserter 2601 changes the length of aguard interval to be added to a valid symbol according to the channelquality. However, based on the decision result above, guard intervalinserter 2601 sets the length of the guard interval to be added to thevalid symbol indicating important information to a predetermined fixedvalue independently of the channel quality. On the other hand, in thecase of the valid symbol indicating normal information, guard intervalinserter 2601 sets a guard interval length according to the channelquality using the method described in Embodiment 1.

FIG. 31 shows an example of the step of the guard interval inserter ofthe OFDM transmission/reception apparatus of the present embodimentadding a guard interval. As shown in the chart, a longer guard intervalis added to the valid symbol indicating important information than theguard interval added to the valid symbol indicating normal information.

FIG. 32 shows an example of the step of guard interval eliminator 111 ofthe OFDM transmission/reception apparatus of the present embodimenteliminating a guard interval. Guard interval eliminator 111 is informedof the timings of starting the guard interval and starting the validsymbol. Therefore, guard interval eliminator 111 can extract only validsymbols and eliminate guard intervals through normal processing even ifthe guard interval length in the valid symbol indicating normalinformation and the valid symbol indicating important informationchanges.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment distinguishes important information from normal informationin a transmission signal and sets the length of a guard interval to beadded to the valid symbol indicating the important information to apredetermined value that implements an error rate lower than the errorrate about the normal information independently of the channel quality.Through this, the OFDM transmission/reception apparatus of the presentembodiment can change the length of a guard interval to be added to thevalid symbol indicating the normal information thereby improving thetransmission efficiency, while it can set a lower error rate forimportant information than for normal information. Thus, the OFDMtransmission/reception apparatus of the present embodiment can improvethe channel quality and transmission efficiency.

Embodiment 19

The OFDM transmission/reception apparatus of Embodiment 19 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 18, and always adds tothe valid symbol indicating important information a guard intervallonger by a certain length than the guard interval to be added to thevalid symbol indicating normal information.

Since the OFDM transmission/reception apparatus of Embodiment 18 alwaysadds a certain length of guard interval to the valid symbol indicatingimportant information, in the case that the error rate is low and thechannel quality is high, a longer guard interval than is necessary isadded.

Thus, in the guard interval length setting in the present embodiment,the OFDM transmission/reception apparatus sets the length of the guardinterval to be added to the valid symbol indicating importantinformation to the length of the guard interval to be added to the validsymbol indicating normal information according to the channel qualityplus a predetermined constant value.

In this way, the OFDM transmission/reception apparatus of the presentembodiment distinguishes important information from normal informationin the transmission signal and always sets the length of the guardinterval to be added to the valid symbol indicating importantinformation longer than the guard interval to be added to the validsymbol indicating normal information by a predetermined constant value.Through this, the OFDM transmission/reception apparatus of the presentembodiment can change the length of a guard interval to be added to thevalid symbol indicating normal information thereby improving thetransmission efficiency, while it can set a lower error rate forimportant information than for normal information. Thus, the OFDMtransmission/reception apparatus of the present embodiment can improvethe channel quality and transmission efficiency.

Embodiment 20

The OFDM transmission/reception apparatus of Embodiment 20 of thepresent invention has the same configuration as that of the OFDMtransmission/reception apparatus of Embodiment 19, and adds to the validsymbol indicating important information a guard interval longer by apredetermined value determined according to the channel quality than theguard interval to be added to the valid symbol indicating normalinformation.

The OFDM transmission/reception apparatus of Embodiment 19 of thepresent invention always adds to the valid symbol indicating importantinformation a guard interval consisting of the guard interval added tothe valid symbol indicating normal information according to the channelquality plus a predetermined constant length. Therefore, under acondition with a high error rate and poor channel quality, there may becases where it is impossible to improve the error rate of importantinformation by predetermined percentage compared to the error rate ofnormal information.

Therefore, in the guard interval length setting of the presentembodiment, the OFDM transmission/reception apparatus sets the length ofthe guard interval to be added to the valid symbol indicating importantinformation to the length of the guard interval to be added to the validsymbol indicating normal information according to the channel qualityplus a predetermined length proportional to the channel quality.

That is, as the channel quality improves and the error rate decreases,the OFDM transmission/reception apparatus of the present embodimentshortens the predetermined value added to the length of the guardinterval to be added to the valid symbol indicating normal information,and to the contrary as the channel quality deteriorates and the errorrate increases, the OFDM transmission/reception apparatus of the presentembodiment lengthens the predetermined value added to the length of theguard interval to be added to the valid symbol indicating normalinformation.

As shown above, the OFDM transmission/reception apparatus of the presentembodiment distinguishes important information from normal informationin the transmission signal and sets the length of the guard interval tobe added to the valid symbol indicating important information longerthan the guard interval to be added to the valid symbol indicatingnormal information by a predetermined variable length according to thechannel quality. Through this, the OFDM transmission/reception apparatusof the present embodiment can change the length of a guard interval tobe added to the valid symbol indicating normal information therebyimproving the transmission efficiency, while it can set a lower errorrate for important information than for normal information. Thus, theOFDM transmission/reception apparatus of the present embodiment canimprove the channel quality and transmission efficiency.

Here, in contrast to the configuration of the OFDMtransmission/reception apparatus of Embodiments 1 to 17 with a variableguard interval length, the configuration of the OFDMtransmission/reception apparatus of Embodiments 18 to 20 abovedistinguishes important information from normal information in thetransmission signal and sets the guard interval length as follows:

1) For normal information: variable according to channel quality; forimportant information: constant

2) For normal information: variable according to channel quality; forimportant information: constant difference from the one “for normalinformation”

3) For normal information: variable; for important information:difference from the one “for normal information” is variable accordingto channel quality

These are intended to keep the error rate of important information lowerthan the error rate of normal information, and the OFDMtransmission/reception apparatus can take any configuration other thanthe above 3 configurations so long as it can achieve these purposes.

Embodiments 18 to 20 above are Embodiment 1 with the additional functionto improve the error rate about important information. In like manner,it is also possible to add the additional function to improve the errorrate about important information to Embodiments 2 to 17 by combiningEmbodiments 18 to 20 above with Embodiments 2 to 17.

Embodiments 18 to 20 above can improve the error rate not only aboutimportant information but also about specific information and packet(burst). Embodiments 18 to 20 above can have a longer guard intervalthan other information or packet (burst) in the case of multicast, forexample.

As explained above, the present invention is capable of improvingtransmission efficiency while maintaining the function of adding guardintervals and eliminating delayed signals.

The present invention is not limited to the above described embodiments,and various variations and modifications may be possible withoutdeparting from the scope of the present invention.

This application is based on the Japanese Patent Application No.HEI10-365430 filed on Dec. 22, 1998 and the Japanese Patent ApplicationNo.HEI 11-074621 filed on Mar. 18, 1999, entire content of which isexpressly incorporated by reference herein.

1. A method of setting a guard interval in an OFDM communication,comprising: attaching a part of a first valid symbol to the first validsymbol as a guard interval, the first valid symbol being normalinformation; attaching a part of a second valid symbol to be transmittedafter the first valid symbol is transmitted, the second valid symbolbeing retransmission information and requiring higher channel qualitythan the first valid symbol, to the second valid symbol as a guardinterval; and providing the guard interval of the second valid symbol ata length greater than the guard interval of the first valid symbol. 2.The method of claim 1, further comprising maintaining the length of thefirst valid symbol and the second valid symbol.
 3. The method of claim1, further comprising: inserting user data in the first valid symbol;and inserting control data in the second valid symbol.
 4. The method ofclaim 1, further comprising changing the length of the guard interval ofthe first valid symbol in accordance with channel quality.
 5. The methodof claim 1, further comprising maintaining the length of the guardinterval of the second valid symbol.
 6. The method of claim 1, furthercomprising maintaining the length of the guard interval of the secondvalid symbol at a predetermined length greater than the guard intervalof the first valid symbol.
 7. The method of claim 1, further comprisingforming the guard interval of the second valid symbol by attaching alength that changes in accordance with channel quality of the guardinterval of the first valid symbol.
 8. A method of setting a guardinterval in an OFDM communication, comprising: attaching a part of afirst valid symbol to the first valid symbol as a guard interval;attaching a part of a second valid symbol to be transmitted after thefirst valid symbol is transmitted, the second valid symbol requiringhigher channel quality than the first valid symbol, to the second validsymbol as a guard interval; and changing the length of the guardinterval of the first valid symbol in accordance with channel qualitywhile maintaining the length of the guard interval of the second validsymbol.
 9. A method of setting a guard interval in an OFDMcommunication, comprising: attaching a part of a valid symbol to thevalid symbol as a guard interval; and providing the guard interval at agreater length when the valid symbol of retransmission informationrequires higher quality.
 10. A method of setting a guard interval in anOFDM communication, comprising: attaching a part of a valid symbol tothe valid symbol as a guard interval; and providing a guard interval ofa valid symbol including retransmission information for control data ata length greater than a guard interval of a valid symbol including userdata.
 11. An OFDM communication apparatus comprising: an attacherconfigured to attach a part of a first valid symbol to the first validsymbol as a guard interval, and further configured to attach a part of asecond valid symbol to the second valid symbol to be transmitted afterthe first valid symbol is transmitted, the second valid symbol requiringhigher channel quality than the first valid symbol as a guard interval;and a provider configured to provide the guard interval of the secondvalid symbol at a length greater than the guard interval of the firstvalid symbol, wherein the first valid symbol is normal information, andthe second valid symbol is retransmission information.